Simplest method of writing syslog messages?
What technology I have to use to receive syslog messages in UF server and write it into a file?
Free version which has almost all features required for Splunk like filtering ect.
Usually rsyslog or syslog-ng are used for that. I guess most linux distributions ship with one of those 2 installed by default as the OS' syslog engine. You can simply add config to spin up one or more extra instances for your syslog data feeds.
Can we setup HA for rsyslog?
Yes, you can set up multiple servers running rsyslog/syslog-ng and put a network load balancer in front of them. Or use tools like keepalived.